A rail workers' union has called for the "disastrous" Tube public private partnership (PPP) to be scrapped - four years after the deal was made.

The Rail Maritime and Transport Union (RMT) said Metronet and Tube Lines, the firms responsible for maintaining the network, had "spectacularly failed".

A government spokesman said that by 2017 PPP will have delivered £16bn of investment for the Tube.

Transport for London (TfL) said it had done all it could to make PPP work.

It is imperative that the government continues to support and finance the renewal of the Underground

TfL spokesperson

A spokesman for the government said that during the course of the contracts almost 250 stations would be refurbished, more than 300 new trains would be brought into service and 80% of the track would be updated.

"We have also provided the Mayor of London with the transport funding needed for the Olympics," he said.

But the RMT said warnings that PPP would turn out to be an "expensive failure" had come true.

It said Metronet and Tube Lines had been given £3.3bn of public money and made £300m in profits "but by anyone's standards... they have failed to deliver".

'Vital improvements'

RMT general secretary Bob Crow said the move had resulted in "deterioration in service, missed targets, infrastructure failure, engineering overruns, an alarming increase in safety problems and a massive increase in costs."

Mr Crow added that the government must "bring the necessary legislation forward to end the PPP and return the Tube's infrastructure to London Underground".

A TfL spokesperson said: "TfL's position on the PPP is clear. It is not the system we would have chosen to finance and manage the renewal of the Tube. But once the government transferred the Underground to TfL under PPP, we have done all we can to make it work.

"What is vital is that the major capacity improvements - the line upgrades - due from 2009 onwards, are delivered by Metronet and Tube Lines on time and on budget.

"It is imperative that the government continues to support and finance the renewal of the Underground."